The killer, who appears in propaganda photos showing the shooting of an alleged spy, may have been chosen by Isil as part of a crude attempt to show it is an "equal opportunities" organisation.

An account of how he murdered a man in a public square in Sirte is given in a report by Human Rights Watch, which says the group has executed up to 49 people since seizing control of the Libyan city in February 2015.

The extremists have imposed a brutal regime on the city, with killings, public floggings and crucifixions ordered by the religious courts. One former resident, identified only as "Ali" for his own safety, told Human Rights Watch a relative named Milad Ahmed Abourgheba was killed by the disabled executioner in January. He had been found guilty by an Isil court of spying. "Milad disappeared for three months, then on January 16, they shot him dead," Ali said.

"The man who shot him was Tunisian and was in a wheelchair."

Photos of the killing show the wheelchair-bound man wearing military fatigues and a balaclava, with a black Isil flag in the background.